Techniques for enhanced inter-cell interference coordination (ICIC) for co-channel Heterogeneous Network deployment are being introduced in Release 10 of LTE in 3GPP. Co-channel Heterogeneous Networks comprise macrocells and small cells operating on the same frequency channel. Such deployments present some specific interference scenarios for which new ICIC techniques are required.
In one scenario, the small cells are picocells, which are open to users of the macrocellular network. In order to ensure that such picocells carry a useful share of the total traffic load, user equipment (UEs) may be programmed to associate preferentially with the picocells rather than the macrocells, for example by biasing the SINR threshold at which they will select a picocell to associate with. Under such conditions, UEs near the edge of a picocell's coverage area will suffer strong interference from one or more macrocells. In order to alleviate such interference, some subframes may be configured as “blank” or “almost blank” in a macrocell. A blank subframe contains no transmission from the macrocell, while an “almost blank” subframe typically contains no data transmission and little or no control signaling transmission, but will contain reference signal transmissions in order to ensure backward compatibility with legacy terminals which expect to find the reference signals for measurements but are unaware of the configuration of almost blank subframes. Almost blank subframes may also contain synchronisation signals, broadcast control information and/or paging signals.
In order to make the use of blank or almost blank subframes (ABSs) effective (note that hereafter the term “ABS” is used, and should be understood to include both blank and almost blank subframes), signaling is needed from the macrocell to the picocell across the corresponding backhaul interface, known in LTE as the “X2” interface. For LTE Release 10, it has been agreed that this X2 signaling will take the form of a coordination bitmap to indicate the ABS pattern (for example with each bit corresponding to one subframe in a series of subframes, with the value of the bit indicating whether the subframe is an ABS or not). Such signaling can help the picocell to schedule data transmissions in the picocell appropriately to avoid interference (e.g. by scheduling transmissions to UEs near the edge of the picocell during ABSs), and to signal to the UEs the subframes which should have low macrocellular interference and should therefore be used for RRM/RLM/CQI measurements. (RRM=Radio Resource Management, typically relating to handover; RLM=Radio Link Monitoring, typically relating to detection of serving radio link failure; CQI=Channel Quality Information, derived from the signal strength from the serving cell and the interference from other cells, and typically used for link adaptation and scheduling on the serving radio link). It should be noted that subframe patterns that are signalled to the UE for different measurements related to RRM, RLM and CSI could be different in terms of pattern and/or periodicity.
In some cases, a picocell may overlap multiple macrocells, and Pico UEs (PUEs) in different parts of the picocell may therefore suffer interference from different macrocells. Each macrocell may use a different ABS pattern. In such a case, CQI measurements made by a PUE will depend on the particular subframes in which the interference is measured. If the CQI measurements are to be useful to the PeNB in determining which subframes to schedule a PUE in and what modulation and coding scheme (MCS) to use in those subframes, the PeNB needs CQI information corresponding to the different ABS patterns of the different macrocells.
Note that a similar scenario can arise with Heterogeneous Network in which the small cells are femtocells, which operate on a Closed Subscriber Group (CSG) basis, and are therefore typically not open to users of the macrocellular network. In this case, the femtocells can cause strong interference to the macrocell UEs when they come close to the femto eNBs. It may then be beneficial for the macrocells to indicate to their UEs the subframes in which they should make resource specific measurements (i.e. the subframes in which interference from one or more femtocells is reduced or absent).
Current versions of the LTE specifications define only a single CQI report, and this is not related in anyway to particular subframes with different levels of interference.